1. Field of the Invention
The present invention relates to a read circuit in a magnetic disk drive using a sector servo method.
2. Description of the Related Art
In a magnetic disk drive such as a floppy disk device or a hard disk device, when data recorded respectively on a plurality of cylinders provided in a concentric manner in a recording medium is read by a magnetic head, voltage peak values of read signals are different depending on cylinder positions. For this reason, according to a conventional method, read signals are amplified by an AGC (Automatic Gain Control) amplifier such that the voltage peak values of read signals read from different cylinders are equal to each other. A read circuit for controlling the gain of the AGC amplifier is arranged as shown in FIG. 1.
As shown in FIG. 1, data recorded on a recording medium 1 is read by a magnetic head 3, amplified by a head amplifier 5, and then input to an AGC amplifier 7. The noise component of a signal amplified by the AGC amplifier 7 is removed by a low-pass filter 9. Thereafter, the signal is input to a peak detection/read pulse generator 11, a sample/hold circuit 15, and an AGC voltage generator 13. The sample/hold circuit 15 detects head position setting data. The peak detection/read pulse generator 11 detects the peak value of the read signal (since data is written such that the peak position is set to be a magnetic reverse position) and generates a read pulse corresponding to the peak position. The read pulse is supplied to, e.g., a host processor through an interface control circuit 21. A main controller 23 outputs a command for determining whether the AGC voltage generator 13 is AGC-controlled (held) depending on a specific sector area which is read by the magnetic head. When the main controller 23 outputs an AGC signal, the AGC voltage generator 13 outputs an AGC voltage to the AGC amplifier 7. The value of the AGC voltage is changed in accordance with the voltage difference between a voltage peak value output from the low-pass filter 9 and a predetermined value, such that the voltage peak value output from the AGC amplifier 13 is adjusted to be the predetermined value. A speed for causing the output signal of the AGC amplifier 13 to reach the predetermined voltage peak value is generally called an AGC tracking speed. The tracking speed is divided into an attack time and a decay time. The attack time represents a time required for amplifying a small amplitude into a large target amplitude, and the decay time represents a time required for amplifying a large amplitude into a small target amplitude.
The AGC tracking speed is generally set to be a proper value (generally between ten and twenty ms) from an external circuit. In a servo surface servo method (also referred to as a dedicated servo method), in order to adjust the tracking position of a magnetic head, only the peak position of a waveform is detected. In a sector servo method, as shown in FIG. 2, the tracking position of a magnetic head is adjusted such that the amplitude value of a burst A signal is equal to the amplitude value of a burst B signal. For this reason, a read circuit of a magnetic disk drive in the sector servo method must strictly respond to an amplitude. Therefore, it is important how the same gain of the AGC amplifier 8 is obtained at the inner and outer peripheral portions of the magnetic disk. According to a conventional method of holding an AGC mode, only one time constant can set until the AGC mode is held. A recording data format in the sector servo method can be roughly divided into a servo area and a data area. The servo area is an area in which a head position setting signal is recorded, and the data area is an area in which data sent from a host processor is recorded. The servo and data areas are alternately arranged. In general, in order to record the maximum number of data in the data area, the data area is designated to be larger than the servo area as possible. For this reason, in the servo area, an AGC tracking speed must be increased to accurately read data written in a very small range.
Data having various frequencies are written in the data area. When low-frequency data is written in the data area after high-frequency data is written therein, the AGC mode is rapidly held to decrease the gain of the high-frequency data. At this time, the AGC mode is held on the low-frequency data, and the data is broken. For this reason, in the data area, the AGC tracking speed is preferably set to be low.
The magnetic characteristic curve of the recording medium has modulation. That is, the recording medium has some portions having small magnetic forces. Since the speed of a magnetic disk is generally three thousands and several hundreds rpm, a time for one rotation is between ten and twenty msec. At this time, when the tracking speed of the AGC amplifier is set to be, e.g., several hundreds .mu.s, no AGC mode may be held on a portion having a small magnetic force.
In the sector servo method, data used as digital data and a head position setting data used as analog data are serially recorded in one sector. Since it is checked that the data section is set to be "1" or "0" at the peak positions of signals, the peak positions must be uniformed by the AGC mode. On the other hand, since the head position setting data, e.g., areas of the bursts A and B shown in FIG. 2, is processed with a 1/2 amplitude, the AGC mode is not preferably held on the position setting data. For this reason, an AGC/HOLD signal serving as a switching signal for performing a switching operation of an automatic gain control operation and a gain holding operation is input from the main controller 23 to the AGC voltage generator 13, thereby reading the recording data serving as digital data and the position setting data serving as analog data serially recorded.
Although the digital data and the analog data can be read by switching the operation modes of the AGC voltage generator 13, when the AGC amplifier 7 is switched into a gain holding mode, the gain of the AGC mode immediately before the AGC amplifier 7 is switched into the gain holding mode and is continuously held. For this reason, when there is a switch in the AGC gain immediately before the AGC amplifier 7, a predetermined target gain cannot be tracked because the AGC gain is held. Therefore, a peak value serving as the analog data may be erroneously detected.